Porphyrins are derivatives of porphine, a conjugated cyclic structure of four pyrrole rings linked through their 2- and 5-positions by methine bridges. Porphyrins can be covalently attached to other molecules. The electronic features of the porphyrin ring system can be altered by the attachment of one or more substituents. The term "porphyrin" includes derivatives wherein a metal atom is inserted into the ring system, as well as molecular systems in which ligands are attached to the metal. The substituents, as well as the overall porphyrin structure, can be neutral, positively charged, or negatively charged.
Numerous porphyrins have been isolated from natural sources. Notable porphyrin-containing natural products include hemoglobin, the chlorophylls, and vitamin B.sub.12. Also, many porphyrins have been synthesized in the laboratory, typically through condensation of suitably substituted pyrroles and aldehydes. However, reactions of this type generally proceed in low yield, and cannot be used to produce many types of substituted porphyrins.
Accordingly, there exists a need in the art for synthetic methods capable of producing a greater variety of porphyrins than presently available.